The present invention relates to a contact-type image sensor with a multiplicity of photoelectric conversion elements disposed in an array, for example, a linear array. The sensor may be used in the reader of a facsimile device (telecopier), or in the image input unit of office automation equipment, a paper money discriminator, a hand scanner, or any of a number of other devices in which an image is to be converted to electrical signals.
When an image is to be sent from one facsimile device to another, the sending device must convert the image to electrical signals, which are transmitted to the receiving device where the image is recreated. It is well known to equip the sending device with a document reader over which the image is passed (or which passes over the image). The document reader is typically an image sensor with a multiplicity of photoelectric conversion elements disposed in one row. In a typical device, these elements may be spaced as closely as 8 elements per millimeter.
Each element responds to light and dark areas in one small portion of the image and thus the image is divided into picture elements, sometimes called "pixels" or "dots".
One example of a well-known type of image sensor is that shown in FIGS. 1(a) and 1(b). A multiplicity of chromium individual electrodes 1 having a connector 2 are formed on an insulating substrate 3 through photoetching; a photoelectric conversion semiconductor film 4 consisting of an intrinsic amorphous silicon (hereinafter called a-Si) is then grown through masking; and finally an ITO (indium-tin-oxide) common electrode 5 forming a hetero junction with the a-Si film 4 is provided through masking.
A second example of a well-known type of image sensor is that shown in FIG. 2. Using photoetching techniques, a multiplicity of conductors 6 and individual electrodes 1 are formed in an array on an insulating substrate 3. Connectors 2 make electrical contact between the conductors 6 and the individual electrodes 1. A connection 7 for attachment to a common electrode is formed on the insulating substrate 3. A semiconductor film 4 consisting of an a-Si film is then grown through masking. Finally an ITO common electrode 5 forming a hetero junction with the a-Si film 4 is provided through masking.
A third example of a well-known type of image sensor is that shown in FIG. 3. Using photoetching techniques, a multiplicity of individual electrodes 1 and connectors 2 are formed of chromium, and joined to wiring 6 which is formed of chromium-gold on the insulating substrate 3. A PIN junction semiconductor film 4 comprising a layer of a P-type hydrogenated amorphous silicon carbide (hereinafter called a-SiC), a layer of an intrinsic a-Si, and a layer of an N-type a-Si deposited in sequence is formed through masking. An ITO common electrode 5 is then formed through masking and finally a chromium douser film 8 is formed thereon through photoetching.
Each of these known image sensors suffers from certain drawbacks. For example, in the example of FIGS. 1(a) and 1(b), since the semiconductor film 4 is in direct contact with the substrate 3 at a plane 9, impurities are able to diffuse from the substrate through the plane to cause deterioration in the characteristics of the semiconductor film. For example, if glass is used as the substrate, impurities such as alkali metals, alkaline earth metals or the like which can alter the characteristics of the semiconductor may diffuse out of the glass. Hence there has heretofore been no alternative but to use an expensive ceramic for the insulating substrate 3, driving up the cost of the image sensor. Further, it is difficult to obtain a satisfactory adhesion between the semiconductor film 5 and the substrate 3, thus leading to a deterioration in reliability.
In the example of FIG. 2, since the common electrode 5 is formed through masking, some variation in the dimensions of the common electrode and its extent of overlap with the individual electrodes is unavoidable. This variation is shown as distance x in FIG. 2. These variations in dimensions gives rise to variations in the effective photoelectric conversion area, thus causing variations in photoelectric output. This turn causes a deterioration of image reading quality and yield.
Special manufacturing processes have been employed to minimize the variation x. These special processes are expensive, however, and thus not entirely acceptable.
Finally, in the example of FIG. 3, the chromium douser film 8, which establishes the pixel area, is formed through photoetching, and thus the lighting window defined by the film must be precisely positioned. For example, a sensor with 8 dots per millimeter has individual electrodes 1 that are 100 .mu.m square and lighting windows established by the douser film 8 that are 60 to 100 .mu.m square. In order for each lighting window to overlap completely with an individual electrode so that the effective photoelectric conversion areas are the same, the lighting window must be positioned with a precision of .+-.20 .mu.m or better. In the case of a large-sized substrate for scanning a document with a width of several tens of centimeters, a precision of .+-.20 .mu.m or better is very hard to obtain, and when photoetching the douser film 8, markers must be matched very strictly, using, for example, a microscope. This and other factors, such as dimensional variation due to contraction and expansion of the substrate, make the manufacturing process quite expensive.
It is therefore an object of the invention to provide a contact image sensor, which is free of the problems discussed in the context of FIGS. 1(a) and 1(b), such as deterioration in characteristics of the pixels due to diffusion of an impurity from the insulating substrate to the semiconductor film. Another object of the invention is to provide satisfactory adhesion between the semiconductor film and the substrate. Yet another object of the invention is, in the example of FIGS. 2 and 3, to provide a contact image sensor wherein the sensor may be manufactured through a simple process, and wherein the variability in photoelectric output of each pixel is minimized.